Optical amplifier for use in optical communications equipment

ABSTRACT

The output level of pumping light supplied from a pumping light source is varied using a variable attenuator, which is controlled by a variable attenuator driver circuit. By separating a portion containing the pumping light source from a portion containing an amplification medium, it becomes possible to prevent heat emitted by the pumping light source from adversely affecting the amplification medium. By arranging the portion containing the pumping light source such that a pumping light source or sources can be added when necessary, optical transmission system requirements of having more pumping light sources for system upgrade can be accommodated readily. By packaging portions containing amplification media, an optical amplifier can be made small.

This appln is a Div of Ser. No. 09/042,790 filed Mar. 17, 1998.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an optical amplifier for use in opticalcommunications equipment.

2. Description of the Related Art

In recent years, wavelength division multiplexing (WDM) transmissionsystems have been introduced. In transmitting stations, increasing thenumber of signal channels results in an increase in the number of postamplifiers used. An optical amplifier takes up a lot of space in opticaltransmitting equipment. It is therefore desired that the opticalamplifier be decreased in size and produced in integrated form.

In the optical amplifier, a laser diode (LD) is used as a pumping lightsource and a component that gives off heat generated by the LD isassociated with the light source. Of components that construct theoptical amplifier, the light source and its associated radiator are verylarge, preventing the optical amplifier from becoming reduced in size.Thus, separating the pumping light source from the optical amplifier andgrouping components necessary for use as a pumping light source, such asa light source and its associated driving circuit, as a pumping lightsource unit were thought of.

FIGS. 1A, 1B and 1C show arrangements of conventional pumping lightsource units.

As shown in FIGS. 1A, 1B and 1C, three types of pumping light sourceunits may be considered (note that the example of FIG. 1A uses fourpumping LDs). The example of FIG. 1C is used in the WDM system developedby CIENA (see catalog “CIENA Multiwave Line Amplifier Block Diagram”).

In FIGS. 1A to 1C, 1 denotes a circuit for driving a pumping lightsource, 2, 3, 4, 5 and 8; pumping light sources, 6; a polarization beamsplitter (PBS), 7; a wavelength division multiplexing (WDM) coupler, 9;an optical splitter for separating pumping light, and 10; an opticalcoupler/splitter for coupling and separating pumping light.

FIG. 1A, each of the four pumping light sources 2, 3, 4 and 5 is driven(for example, supplied with current) by a respective one of the fourpumping light driving circuits 1 to produce light. The light from eachof the pumping light sources 2 to 5 is output as linearly polarizedlight. The beams of light output from the pumping light sources 2 and 3can be set substantially equal to each other in wavelength but will bedifferently polarized, The beams of light which are substantially equalto each other in wavelength but differently polarized arepolarization-coupled by the polarization beam splitter 6. The pumpinglight sources 4 and 5 are of the same type as the pumping light sources2 and 3 but will differ from the pumping light sources 2 and 3 in outputwavelength. This indicates that, even if the pumping light sources 4 and5 are of the same type as the pumping light sources 2 and 3, theirwavelengths do not necessarily match because of variations inmanufacturing process. The polarization-coupled pumping light from thepumping light sources 2 and 3 and the polarization-coupled pumping lightfrom the pumping light sources 4 and 5 are coupled by the WDM coupler 7.The light output from the WDM coupler is sent to an opticalamplification medium, for example, an erbium doped fiber (EDF), for useas pumping light for amplifying light signals. The WDM coupler is sonamed because it couples wavelength-multiplexed signal light and pumpinglight of a specific wavelength, but in practice it can be an ordinaryoptical coupler.

This arrangement is used when only one pumping light source cannotprovide a sufficient amplification action to the optical amplificationmedium and intended to obtain pumping light with a larger power throughthe use of two or more pumping light sources.

The pumping light source unit of FIG. 1B comprises one pumping lightsource driver 1, one pumping light source 8, and an optical splitter 9for separating pumping light from the pumping light source 8. Thisarrangement is used when the pumping light source has a power largeenough to supply two or more optical amplification media (not shown).This arrangement allows the two or more optical amplification media tobe operated equally with one pumping light source having a singlewavelength and a single polarized wave.

The pumping light source unit of FIG. 1C comprises two or more pumpinglight source drivers 1, an equal number of pumping light sources 8, andan optical coupler/splitter 10 that couples and separates pumping beamsof light from the pumping light sources. This arrangement is intended tosupply two or more optical amplification media with pumping light from asingle pumping light source unit, but it has two or more pumping lightsources 8 to provide pumping light with higher power because a singlepumping light source alone cannot give all of the optical amplifiers asufficient amplifying action. As described above, however, there arevariations in wavelength between two or more pumping light sources 8.Thus, the use of each of the pumping light sources 8 for a respectiveone of the optical amplification media will cause their respectiveamplification actions to vary. For this reason, this pumping lightsource unit is arranged such that beams of pumping light from thepumping light sources 8 are first coupled to produce a single beam oflight and then the single beam of light is separated to thereby supplyeach of the optical amplification media with pumping light of the sameproperty.

Hereinafter, what includes a pumping light source unit, an opticalamplification medium or media, and other circuits including a controlcircuit shall be called an optical amplifier.

In an optical amplifier, automatic gain control (AGC) or automatic levelcontrol (ALC) is sometimes performed to control the supply amount ofpumping light to an optical amplification medium. In conventionaloptical amplifiers, the amount of output light of a pumping light sourceis varied to vary the supply amount of pumping light to the opticalamplification medium by changing a drive current to the pumping lightsource.

In the WDM transmission system, signal light beams of differentwavelengths are collectively amplified by an optical amplifier. Afterthe start of system implementation, the WDM transmission system issometimes modified to increase the signal multiplexing degree (i.e., thesystem is upgraded). When the multiplexing degree is increased, theoptical amplifier requires more pumping power in order to increase thesupply amount of pumping light to the optical amplification media.

In the pumping light source units shown in FIGS. 1B and 1C, the ratio ofthe output power of each pumping light is fixed to the ratio of thesplitting by the splitter 9 or the optical coupler/splitter 10, and theamount of pumping light at each output port cannot be variedarbitrarily.

The optical amplifier contains components that are associated with apumping light source to radiate heat generated by it. These componentsare relatively large among components composing the optical amplifier,preventing the optical amplifier from becoming reduced in size. When LDsas pumping light sources that generate heat and driver circuits thereforare placed close together within the optical amplifier, it will createan excessive rise in temperature, reducing the performance andreliability of the optical amplifier.

Heretofore, even if the WDM transmission system is upgraded, pumpinglight can only be output up to the allowable maximum output of a pumpinglight source installed in an optical amplifier at the beginning ofsystem implementation. As an example, assume that, in a 16-channel WDMtransmission system, only four channels are employed at the beginning ofsystem implementation. In this case, the optical amplifier used isnaturally equipped with pumping light sources necessary to accommodate16 channels, which increases the initial investment at the time ofsystem installation.

In an optical communications device equipped with an optical amplifierof the built-in pumping light source type, heat generated in the deviceis difficult to radiate and it is therefore necessary to cool the devicewith a fan, dissipating extra power.

When pumping light of a narrow spectrum width emitted by an LD as apumping light source reflects from optical parts composing an opticalamplifier or a fiber junction back to the pumping LD, the operation ofthe LD becomes unstable, which makes the operation of the opticalamplifier unstable. To avoid this problem, conventionally the opticalamplifier has an optoisolater built in on the output side of the pumpingLD. This arrangement requires more optical parts.

In the most used type of an optical amplifier, the amplificationcharacteristic of an amplification medium is wavelength-dependent, andpumping LDs have variations in wavelength due to variations in themanufacturing process. For this reason, the optical amplifier hasvariations in amplification characteristic due to variations inwavelength.

In an optical amplifier in which a pumping light source unit and anoptical amplification medium are coupled together by means of aconnector, it is necessary to sound an alarm in the case that theconnector has come off, because pumping light leaking out through theconnector is very dangerous for persons at work.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide an optical amplifierwhich has a function of changing the supply amount of pumping light toan amplification medium, and is small in size and little affected byheat generated by a pumping light source.

According to a first aspect of the present invention, there is providedan optical amplifier for amplifying incoming signal light in response topumping light applied thereto, characterized by including a variableattenuator for varying the input level of the pumping light applied tothe optical amplifier to thereby tune the amplification characteristicof the optical amplifier.

In the optical amplifier of the present invention, two or more opticalamplification units that contain amplification media are assembled intoone package.

The optical communications device of the present invention comprises apumping light source unit having at least one pumping light source forgenerating pumping light and a unit for separating or coupling pumpinglight from the at least one pumping light source, and an opticalamplification unit for amplifying incoming signal light in response toapplication thereto of pumping light from the pumping light source unitand is characterized in that the pumping light source unit is placed ina location in the optical communications device where heat radiationconditions are good.

According to a second aspect of the present invention, there is providedan optical amplifier for amplifying incoming signal light in response toapplication thereto of pumping light from a pumping light source unithaving a pumping light source for generating a pumping light beam and anoptical coupler unit for coupling multiple pumping light beams,characterized in that the pumping light source unit includes apolarization plane rotating unit for rotating the plane of polarizationof output pumping light from the optical coupler unit through a firstangle of rotation for transmission and rotating the plane ofpolarization of return light, resulting from the output pumping lightbeing reflected from a connector connecting the pumping light sourceunit and other components of the optical amplifier back to the pumpinglight source unit, through a second angle of rotation, thereby inputtingto the pumping light source return light, different in wavelength fromthe pumping light source generated by the pumping light source.

According to a third aspect of the present invention, there is providedan optical amplifier for amplifying incoming signal light in response toapplication thereto of pumping light from a pumping light source unithaving multiple pumping light sources each generating a pumping lightbeam and an optical coupler/splitter unit for coupling multiple pumpinglight beams and splitting into individual light beams, characterized inthat the pumping light source unit includes a polarization planerotating unit for rotating the plane of polarization of output pumpinglight from the optical coupler unit through a first angle of rotationfor transmission and rotating the plane of polarization of return light,resulting from the output pumping light being reflected from a connectorconnecting the pumping light source unit and other components of theoptical amplifier together back to the pumping light source unit,through a second angle of rotation, thereby inputting to the pumpinglight source return light different in wavelength from the pumping lightsource generated by the pumping light source.

According to a fourth aspect of the present invention, there is providedan optical amplifier in which a pumping light source unit having apumping light source for generating pumping light and an opticalamplification unit having an amplification medium for amplifyingincoming signal light in response to application of the pumping lightthereto are connected together by means of a connector that allows thepumping light to be transmitted to the optical amplification unit,characterized in that the optical amplification unit includes a unit fordetermining whether or not the connection between the pumping lightsource unit and the optical amplification unit is established by meansof the connector on the basis of the output level of the pumping lightfrom the pumping light source unit.

According to a fifth aspect of the present invention, there is providedan optical amplifier in which a pumping light source unit having apumping light source for generating pumping light and an opticalamplification unit having an amplification medium for amplifyingincoming signal light in response to application of the pumping lightthereto are connected together by means of a connector that allows thepumping light to be transmitted to the optical amplification unit,characterized in that the pumping light source unit includes a unit fordetermining whether or not the connection between the pumping lightsource unit and the optical amplification unit is established by meansof the connector on the basis of the level of return light reflectedfrom the connector.

According to a sixth aspect of the present invention, there is providedan optical amplifier in which a pumping light source unit having apumping light source for generating pumping light and an opticalamplification unit having an amplification medium for amplifyingincoming signal light in response to application of the pumping lightthereto are connected together by means of a connector that allows thepumping light to be transmitted to the optical amplification unit,characterized by comprising a unit for determining whether or not theconnection between the pumping light source unit and the opticalamplification unit is established by means of the connector.

An optical amplification unit of the present invention has anamplification medium for amplifying incoming signal light in response toapplication thereto of pumping light from a separate pumping lightsource unit, the optical amplification unit and the pumping light sourceunit being connected by a connector to form an optical amplifier, and ischaracterized by the provision of a variable attenuator for adjustingthe level of pumping light input to the amplification medium.

A pumping light source unit of the present invention has a pumping lightsource for generating pumping light to be output to a separate opticalamplification unit, the pumping light source unit and the opticalamplification unit being connected by a connector to form an opticalamplifier, and is characterized by the provision of a variableattenuator for adjusting the level of pumping light to be output to theoptical amplification unit.

According to the present invention, the pumping light, which, in theconventional system, has its output level maintained constant oradjusted by controlling the pumping light source itself, can belevel-adjusted easily by the use of the variable attenuator, whichallows pumping light with a suitable intensity to be supplied to theamplification medium.

In addition, since the pumping light source is separated from theoptical amplification unit, two or more optical amplification units canbe grouped into one package, ensuring compactness of the opticalamplifier.

Being separated from the optical amplification unit, the pumping lightsource can be placed in a location where heat radiating conditions aregood and can suppress the effect of heat on the light amplificationunit.

By the provision of the unit for rotating the plane of polarization ofpumping light output from the pumping light source through apredetermined angle in the pumping light source unit, the operationalinstability of the light source due to return light can be eliminated.

According to the present invention, the optical amplifier is separatedinto the pumping light source unit containing the pumping light sourceand the optical amplification unit containing the amplification mediumand a connector is therefore required to couple these units together.The connector may come off while a person is working. Not only has thepumping light high power, but it is converged by an optical fiber. Inthe event that the connector has come off, therefore, the person at workmay be exposed to danger. To avoid such danger, the optical amplifier ofthe present invention is provided with means for detecting whether theconnector is off or not.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A, 1B and 1C show arrangements of conventional pumping lightsource units;

FIG. 2 illustrates the principle of the present invention;

FIG. 3A shows an arrangement of a variable attenuator;

FIG. 3B shows an arrangement of the Faraday rotator of FIG. 3A;

FIG. 4 shows a first embodiment of an optical amplifier of the presentinvention;

FIG. 5 shows a second embodiment of the optical amplifier of the presentinvention;

FIG. 6 shows a third embodiment of the optical amplifier of the presentinvention;

FIG. 7 shows a first arrangement of the pumping light source unit;

FIG. 8 shows a second arrangement of the pumping light source unit;

FIG. 9 shows a third arrangement of the pumping light source unit;

FIG. 10 shows a fourth arrangement of the pumping light source unit;

FIG. 11 shows a fifth arrangement of the pumping light source unit;

FIG. 12 shows a sixth arrangement of the pumping light source unit;

FIG. 13 shows a seventh arrangement of the pumping light source unit;

FIG. 14 is a schematic representation of the optical amplifier;

FIG. 15 shows a first arrangement of the optical amplification unit;

FIG. 16 shows a second arrangement of the optical amplification unit;

FIG. 17 shows a third arrangement of the optical amplification unit;

FIG. 18 shows a fourth arrangement of the optical amplification unit;

FIG. 19 shows a fifth arrangement of the optical amplification unit;

FIG. 20 shows an arrangement of the pumping light source unit adapted tooperate the pumping light source stably;

FIG. 21 shows a fourth embodiment of the optical amplifier of thepresent invention;

FIG. 22 shows an arrangement adapted to monitor connection between theoptical amplification unit and the pumping light source unit; and

FIGS. 23A and 23B are diagrams for use in explanation of how to connecta pumping light source or sources in the pumping light source unit.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring now to FIG. 2, there is illustrated the principle of thepresent invention.

An arrangement of an optical amplifier, with a pumping light sourceexcluded, is illustrated here. Hereinafter, the other portion of anoptical amplifier than a pumping light source is referred to as anoptical amplification unit.

Reference numeral 11 denotes an amplification medium that amplifiessignal light by being injected with pumping light, 12; a coupler thatcouples the signal light and the pumping light, 13; a variableattenuator that varies the supply amount of the pumping light to theamplification medium, and 14; a driver that drives the variableattenuator.

In the present invention, signal light and pumping light having itsoutput level adjusted by the variable attenuator 13 driven by the driver14 are input to the coupler 12, then coupled and transmitted to theamplification medium 11. The amplification medium consists of, forexample, an erbium doped fiber. By the pumping light, the amplificationmedium (fiber amplifier) 11 is excited to initiate stimulated emission,thereby amplifying the signal light.

With the arrangement of FIG. 2, even if the light multiplexing degreeincreases, sufficient light amplification can be achieved by merelyadjusting the output level of the pumping light with the variableattenuator 13.

In addition, control of the output power of the pumping light throughthe use of the variable attenuator 13 eliminates the need of controllinga pumping light source itself and moreover allows the pumping lightsource and the optical amplification unit to be arranged separately.Separating the heat emitting pumping light source from the opticalamplification unit susceptible to heat ensures that the opticalamplifier operates stably.

FIGS. 3A and 3B illustrate an example of the variable attenuator.

FIG. 3A illustrates the entire arrangement of the variable attenuator.

The variable attenuator comprises a lens 20 which collimates incominglight from an optical fiber 25, a birefringent wedge 21 which splits anincoming light beam into two components, depending on a difference inpolarization, a variable Faraday rotator 22 which can vary the angle ofFaraday rotation, a birefringent wedge 23 which splits an incoming lightbeam into two components, and a lens 24 which focuses light emergingfrom the birefringent wedge 23. The light transmitted through the lens24 is directed to an optical fiber 26, in which case, depending on theFaraday rotation angle in the Faraday rotator 22, there are two cases;the case where 100% of light is directed to the optical fiber, and thecase where only a part of light is directed. When only a part of lightis directed to the optical fiber, the intensity of light directed to thefiber is attenuated. On the other hand, when 100% of light is directedto the fiber, the intensity of light reaches the maximum level.

Thus, by controlling the Faraday rotation angle of the Faraday rotator,the degree to which a light path is bent in the birefringent wedge 23 isvaried, allowing the intensity of light directed to the optical fiber 26to be controlled.

FIG. 3B illustrates an example of the variable Faraday rotator.

This Faraday rotator comprises a permanent magnet 27, an electromagnet28, a magneto-optical crystal 29, and a variable current source forsupplying current to the electromagnet. A light beam 31 incident on themagneto-optical crystal 29 has its plane of polarization rotated by amagnetic field produced within the magneto-optical crystal by thepermanent magnet 27 and the electromagnet 28. The direction of themagnetic field produced within the magneto-optical crystal changes withthe magnitude of a magnetic field produced by the electromagnet. Theangle of rotation of the plane of polarization of the light beam 31 isdetermined by that component of the magnetic field which is parallel tothe direction in which the light beam travels. Thus, the angle ofrotation of the plane of polarization of the light beam 31 can bechanged by changing the direction of the magnetic field within themagneto-optical crystal 29 with its magnitude unchanged. Here, thepermanent magnet 27 is used to saturate the magnetic field within themagneto-optical crystal 29. By saturating the magnetic field within themagneto-optical crystal, the magnitude of the internal magnetic fieldcan be kept unchanged even if any magnetic field is produced by theelectromagnet 28. By changing the direction of the internal magneticfield, the magnitude of the magnetic field component parallel to thedirection in which the light beam 31 travels is increased or decreased.Thereby, the angle of rotation of the plane of polarization of the lightbeam 31 is controlled.

FIG. 4 illustrates a first embodiment of the optical amplifier of thepresent invention.

This optical amplifier comprises a pumping light source unit 40 and anoptical amplification unit 41. The pumping light source unit couplespumping light emitted from two or more pumping light sources 43 in acoupler 42 for transmission to the optical amplification unit 41. Theprovision of two or more light sources allows a pumping light beam oflarge power to be transmitted to the optical amplification unit 41. Ifthe pumping light source unit 40 is arranged such that a pumping lightsource or sources can be added, the optical amplifier requirements ofhaving higher-power pumping light at a later time will be accommodated.

The optical amplification unit 41 is equipped with a WDM coupler 44 thatcouples incoming signal light and pumping light for transmission to anamplification medium 46 via an optical isolator 45. The amplificationmedium 46 is excited by the pumping light to amplify the light signal,which in turn is directed to an optical isolator 47. The amplifiedsignal is then input to a wavelength selective filter 48 where only thesignal is derived and then input to an optical splitter 49. When thesignal light is amplified in the amplification medium 46 by the use ofthe pumping light, which emit signal light in the opposite direction tothe transmission direction. The optical isolators 45 and 47 are providedto prevent the opposite direction signal light.

In the optical splitter 49, most of the signal light propagatesstraight, but part of the signal light branches off and its total outputis detected by a photodiode 50. The output of the photodiode is input toa monitor circuit 51 to make a decision of whether or not the amplifiedsignal light has reached a predetermined output level. If the decisionis that the amplified signal light has not reached the predeterminedoutput level, then the monitor circuit sends a signal to an attenuatordriver circuit 52. In response to this signal, the driver circuitcontrols the variable attenuator 53 to thereby adjust the pumping lightoutput to the fiber amplifier 46. By using such a feedback loop, theoutput level of the amplified signal light can be maintained constant.The monitor circuit 51 may be formed of a differential amplifier.

In this arrangement, being separated from the pumping light sources 43grouped as a pumping light source unit, the optical amplification unit41 will not suffer from heat emitted by the pumping light sources. Inaddition, unlike the conventional system, the pumping light output levelis adjusted by the variable attenuator 53. Thus, even if the signalmultiplexing degree in signal light is changed, a required output levelof pumping light can be obtained readily by controlling the variableattenuator 53.

FIG. 5 illustrates a second embodiment of the optical amplifier of thepresent invention.

In this figure, the same reference numerals are used to denote partscorresponding to those in FIG. 4 and their descriptions are omitted.

The second embodiment of FIG. 5 is arranged such that a single lightsource 62 can provide sufficient pumping light to each of two or moreamplification media 70 and 46. A pumping light source unit 60 comprisesthe pumping light source 62 and an optical splitter 63 that splitspumping light from the pumping light source for transmission to theamplification media 70 and 46. An optical amplification unit 61comprises two or more amplifiers (first and second amplification units74 and 75). The arrangement of the second optical amplification unit 75is the same as that described in conjunction with FIG. 4 and hence itsdescription is omitted here.

The first optical amplification unit 74 takes a configuration calledbidirectional excitation in which pumping light is input to theamplification medium 70 from its both ends. Two pumping light beams fromthe pumping light source unit 60 are directed to the first opticalamplification unit 74 where their output levels are respectivelyadjusted by variable attenuators 64 and 67 driven by driver circuits 65and 66 and then coupled with signal light by WDM couplers 68 and 71. Thetwo pumping light beams are input to the amplification medium 70 fromits both ends to thereby amplifying the signal light. Optical isolators69 and 72 are provided, as described previously, so as to removespurious light that propagates in the opposite direction to thedirection in which the signal light propagates. The light transmittedthrough the optical isolator 72 is input to a wavelength selectiveoptical filter 73 where only the main signal light is extracted fortransmission over a transmission path (not shown).

In the first optical amplification unit 74, unlike the second opticalamplification unit 75, no feedback is used to maintain the output levelof amplified signal light constant. Of course, a feedback arrangementmay be provided in the first optical amplification unit 74 as well. Inthis case as well, the feedback arrangement is such that the outputlevel of a fraction of main signal light emerging from the wavelengthselective optical filter 73 is detected through the use of a photodiodeand a monitor circuit and the variable attenuators 64 and 67 arecontrolled by the driver circuits 65 and 66 supplied with an outputsignal of the monitor circuit.

The first optical amplification unit 74 uses bidirectional excitation,which is effective in using a very long erbium-doped fiber for theamplification medium 70. That is, in such a case, pumping light inputfrom the WDM coupler 68 to an end of the amplification medium 70 will bedissipated before it reaches the other end of the medium. By inputtingpumping light from the WDM coupler 71 to the other end of theamplification medium as well, therefore, the pumping light is allowed tospread over the entire medium. According to this scheme, even if theamplification medium 70 consists of a very long erbium-doped fiber, theentire medium can be used to amplify signal light.

FIG. 6 shows a third embodiment of the optical amplifier of the presentinvention.

In FIG. 6, as in FIG. 5, the optical amplification unit 61 comprises thefirst and second optical amplification units 74 and 75 and likereference numerals are used to denote corresponding parts to those inFIG. 5.

Using bidirectional excitation, the first optical amplification unit 74in the optical amplification unit 61 is arranged such that pumping lightis directed to both ends of the amplification medium 70. The advantageof this scheme has been described previously. The second opticalamplification unit 75, arranged identically to that of FIG. 4, uses onlyone pumping light beam. As with the second optical amplification unit75, in the first optical amplification unit 74 feedback control may beperformed on the variable attenuators 64 and 65.

In FIG. 6, a pumping light source unit 80 containing pumping lightsources and the optical amplification unit 61 are provided separatelyand, in the optical amplification unit 61, pumping light from thepumping light source 80 to the amplification media 70 and 46 iscontrolled by the variable attenuators 64, 67 and 53. Such anarrangement prevents heat emitted by the pumping light sources 81 fromadversely affecting the stable operation of the optical amplificationunit 61 and allows the required pumping light to be obtained only bycontrolling the variable attenuators 64, 67 and 53. In addition, byarranging the pumping light source unit so as to allow a pumping lightsource or sources to be added to its optical coupler/splitter 82, arequirement of providing such high-power pumping light as is notobtainable from the previously installed pumping light sources 81 can beaccommodated simply by adding a pumping light source or sources to thepumping light source unit 80.

The pumping light source unit 80 of FIG. 6 is equipped with two or morepumping light sources 81 to thereby provide an intense pumping lightthat is not obtainable from a single pumping light source. That is,lights output from the pumping light sources 81 are coupled and thensplit by an optical coupler/splitter 82 for transmission to theamplification media 70 and 46. As described previously, by couplinglights from two or more pumping light sources and then splitting themfor transmission to two or more amplification media, there is providedan advantage that all the amplification media can operate withsubstantially the same characteristics. That is, variations in operatingcharacteristics between amplification media due to variations in amanufacturing process among pumping light sources, which would occur ifone pumping light source were allocated for one amplification medium,can be eliminated.

FIG. 7 illustrates a first arrangement of the excitation light sourceunit.

The pumping light source unit of FIG. 7 is provided with two or morepumping light sources 90 to supply one amplification medium with ahigh-power pumping light which is not obtainable from a single pumpinglight source. Unlike the previous arrangement, in this arrangement avariable attenuator 93 and an attenuator driver circuit 92 are installedin the pumping light source unit as opposed to the optical amplificationunit. In this case, in order to adjust the output level of the pumpinglight in response to the output level of signal light amplified by theamplification medium, it is required to input a feedback control signalto the driver circuit 92 from the optical amplification unit (notshown). As described previously, since the pumping light source unit andthe optical amplification unit are assembled as separate units,electrical wiring is needed to input the feedback control signal to theattenuator driver circuit 92. Although the feedback control arrangementwill not be particularly mentioned below, it will be apparent to thoseskilled in the art.

FIG. 8 shows a second arrangement of the pumping light source unit.

In this arrangement as well, the pumping light source is equipped withvariable attenuators 97 and an attenuator driver circuit 96. Thisarrangement is suitable for a case where a single pumping light source94 has an output high enough to provide pumping light to each of two ormore amplification media (not shown). An optical splitter 95 splitslight from the pumping light source 94. The variable attenuators 97 eachadjust the output level of corresponding pumping light from the opticalsplitter. This is intended to tune each of the amplification mediaindividually. Although, in FIG. 8, the variable attenuators 97 arecontrolled in common by the one driver circuit 96, one driver circuitmay be provided for each variable attenuator. For pumping light outputcontrol feedback, a monitor circuit is provided to detect the outputlevel of signal light amplified by an amplification medium to produce acontrol signal and the control signal is then applied to the attenuatordriver circuit 96 connected to the monitor circuit.

FIG. 9 shows a third arrangement of the pumping light source unit.

In this arrangement, lights from two or more pumping light sources 98are first coupled and then split by an optical coupler/splitter 99. Thisis intended to obtain pumping light of a required output level bycoupling the two or more pumping light sources 98 and to preventvariations in wavelength among lights to the amplification media byfirst coupling all lights from the pumping light sources into one lightbeam and then dividing it. The division of one light eliminatesvariations in wavelength among light beams sent to the amplificationmedia, allowing each amplification medium to operate uniformly. In thearrangement of FIG. 9 as well, variable attenuators 101 and anattenuator driver circuit 100 are provided in the pumping light sourceunit. The variable attenuators 101 are each provided for a light outputof the optical splitter and controlled by the driver circuit 100. Asdescribed previously in conjunction with FIG. 8, an attenuator drivercircuit may be provided for each of the variable attenuators. Forpumping light output feedback control, a control signal is applied froma monitor circuit not shown to the attenuator driver circuit 100.

In the arrangements of FIGS. 8 and 9, pumping light divided by theoptical splitter 95 or optical coupler/splitter 99 is power-controlledby each variable attenuator 97 or 101. This is equivalent to varying thedividing ratio in the splitter or coupler/splitter.

FIG. 10. shows a fourth arrangement of the pumping light source unit.

This pumping light source unit is arranged such that lights from two ormore pumping light sources 110 are coupled by an optical coupler 111into one for application to a single amplification medium and moreoverthe number of light sources connectable to the coupler can be increasedor decreased. For example, the optical coupler 111 has light sourceconnectors and optically coupling components built in which are largerin number than pumping light sources required at optical transmissionsystem startup, and a minimum required number of pumping light sourcesis connected at the time of system installation. When there arises aneed to increase the optical multiplexing degree in the opticaltransmission system (for system upgrade), an additional pumping lightsource or sources (including a separate pumping light source unit orunits) are coupled to the connectors of the optical coupler 111 whichhave been provided in advance. That is, when the optical multiplexingdegree of the optical transmission system increases, the power of signallight increases accordingly. In this case, when the power of pumpinglight remains unchanged, the gain of the fiber amplifier drops. It isthus required to elevate the output level of the pumping light. For thisreason, the pumping light source unit is arranged to allow a pumpinglight source or sources to be added. This avoids the need to installmore pumping light sources than is necessary at the stage of initialinvestment. In addition, higher-power pumping light requirements at thetime of system upgrade can be accommodated readily.

FIG. 11 shows a fifth arrangement of the pumping light source unit.

In the arrangement of FIG. 11, lights from two or more pumping lightsources 110 are first coupled and then separated for transmission toeach amplification medium. In this case as well, an opticalcoupler/splitter 113 is provided beforehand with connectors and opticalelements which allow an additional pumping light source or sources to beadded. When higher-power pumping light becomes necessary for systemupgrade, an upgrading pumping light source or a separate pumping lightsource unit can be connected to the optical coupler/splitter 113.

As shown in FIG. 10 or FIG. 11 since the optical coupler 111 or opticalcoupler/splitter 113 is formed to allow for connection of additionalpumping light source or sources, a minimum required number of pumpinglight sources suffices at system startup. For system upgrade, a pumpinglight source or sources have only to be added to obtain pumping light ata required level. Thus, the initial investment in the opticaltransmission system can be controlled to a minimum and the system can beupgraded readily.

FIG. 12 shows a sixth arrangement of the pumping light source.

This arrangement contains a variable attenuator 115 and an attenuatordriver circuit 114 in addition to the arrangement of FIG. 10. Although,in the arrangement of FIG. 10, the variable attenuator 115 and theattenuator driver circuit 114 are provided in the optical amplificationunit not shown, they may be provided in the pumping light source unit asshown in FIG. 12. This arrangement corresponds to the arrangement ofFIG. 7 but differs in that the optical coupler 111 is formed to allowfor addition of a pumping light source or sources. As described inconjunction with FIG. 7, to feedback control the pumping light outputfrom the optical coupler 111, wiring is needed to input a control signalto the attenuator driver circuit 114.

FIG. 13 shows a seventh arrangement of the pumping light source unit.This arrangement, corresponding to the arrangement of FIG. 11, hasvariable attenuators 116 each corresponding to a respective one ofpumping light outputs of an optical coupler/splitter 113 and anattenuator driver circuit 117 for driving the attenuators. Thisarrangement allows pumping light output to each amplification medium tobe controlled individually. This is substantially equivalent to varyingthe light dividing ratio in the optical coupler/splitter 113.

The arrangement of FIG. 13, which is adapted to supply pumping light tomultiple amplification media, can meet situations where the opticalmultiplexing degree is increased only for some of transmission paths andthe pumping light output level need not be elevated for all theamplification media that the pumping light source accommodates. That is,with system upgrade, pumping light from a pumping light source 112 orseparate pumping light source unit is input to the opticalcoupler/splitter 113 and the light attenuation level in each variableattenuator 116 is set individually. More specifically, pumping light ofthe same output level as prior to the upgrade is applied toamplification media for which the pumping light output level does notneed to be elevated, while pumping light of a higher output level isapplied to amplification media for which the pumping light output levelneeds to be elevated. Thus, the arrangement of FIG. 13 can accommodatevarious system upgrades.

As described previously, in the arrangement of FIG. 13, one attenuatordriver circuit may be provided for each variable attenuator. Forfeedback control of the output level of each pumping light, a controlsignal is applied from a monitor circuit, not shown, to the attenuatordriver circuit.

FIG. 14 is a schematic of an optical amplifier.

The optical amplifier of FIG. 14 is arranged such that a pumping lightsource unit 120 is independent of an optical amplification unit and manyoptical amplification units are incorporated into a package 121. Byincorporating optical amplification units 122-1 to 122-n into onepackage in this manner, optical amplifiers on multiple lighttransmission paths can be integrated, which will not take up too muchspace in the optical communications device. The pumping light sourceunit 120 is installed independently of the optical amplifier unitintegrating package 121, which provides greater freedom in placement ofthe pumping light source unit in the optical communications device.Thus, the pumping light source unit 120, which emits a large amount ofheat and may adversely affect the amplification media in the opticalamplification units, can be placed in a cooling-efficient locationwithin the optical communication device e.g., in the upper portion ofthe device or near to a fan.

To provide an optical amplifier with uniform characteristics, opticalamplification units of the same type are connected to the pumping lightsource unit, For this reason, pumping lights having the same wavelengthor the same wavelength component are input to the amplification media inthe optical amplification units, which allows the optical amplifier tohave uniform characteristics regardless of the wavelength dependence ofthe amplification media.

FIGS. 15 to 18 show various arrangements of the optical amplificationunit described in conjunction with FIGS. 4 and 5.

FIG. 15 shows a first arrangement of the optical amplification unit.

In FIG. 15, 131 denotes an amplification medium (fiber amplifier)consisting of an erbium-doped fiber, 132; a WDM coupler for couplingpumping light and signal light, 133; an optical isolator for preventingoscillation of light propagating in the reverse direction, 134; awavelength selective optical filter which allows signal light wavelengthcomponents to pass through, 135; a variable attenuator which varies thesupply amount of pumping light to the amplification medium, 136; anattenuator driver circuit for supplying a drive current to the opticalattenuator, 137; an optical splitter for branching part of signal lightemerging from the filter for the purpose of controlling the output levelof the pumping light, 138; a photodiode for converting signal lightbranched by the optical splitter into an electrical signal, and 139; acontrol circuit which receives the electrical signal and performsnecessary calculations to send a control signal to the attenuator drivercircuit.

The optical amplification unit of FIG. 15 performs automatic levelcontrol (ALC) to keep the level of output light constant and has thesame arrangement as that described in conjunction with FIG. 4. Part ofsignal light is branched by the optical splitter 137 and then convertedinto an electrical signal, which, in turn, is compared in magnitude witha preset value in the control circuit 139. The ALC is performed bycontrolling the drive current to the variable attenuator 135 via thedriver circuit 136 so that a difference between the electrical signaland the preset value will reach zero.

Separating the optical amplifier into the optical amplification unitcontaining the amplification medium susceptible to heat and the pumpinglight source unit containing a pumping light source or sources allowsthe amplification medium to operate stably and the pumping light sourceunit to have facilities for system upgrades as described previously.Further, unlike the conventional amplifier in which the output level ofpumping light is controlled by varying current to a pumping lightsource, the present invention performs the pumping light level controlby varying the power of pumping light to the amplification medium usingthe variable attenuator while keeping the output level of-the pumpinglight source constant. This makes it possible to form an opticalamplifier from separate units: an optical amplification unit and apumping light source unit.

FIG. 16 shows a second arrangement of the optical amplification unit.

In this figure, like reference numerals are used to denote correspondingparts to those in FIG. 15 and their descriptions are omitted.

In FIG. 16, 140 denotes an optical splitter which branches part ofincoming signal light to control the output level of pumping light, 141;a photodiode which converts the signal light branched by the opticalsplitter into an electrical signal, and 142; a control circuit whichreceives the electrical signals produced by the photodiodes 138 and 141and performs necessary calculations to produce a control signal for theattenuator driver circuit 136.

This amplification unit is arranged to perform automatic gain control(AGC) to keep the signal light gain constant. Part of output signallight branched by the splitter 137 is converted into an electricalsignal by the photodiode 138, while part of incoming signal lightbranched by the optical splitter 140 is converted into an electricalsignal by the photodiode 141. These electrical signals are applied tothe control circuit 142 where the ratio of output signal light to inputsignal light, i.e., the gain, is obtained. This gain is compared with apreset value to produce a difference therebetween. The AGC control isperformed by controlling current to the variable attenuator 135 via thedriver circuit 136 so that the difference will reach zero.

The WDM coupler 132, the optical isolator 133, the wavelength selectivefilter 134 and the amplification medium 131 operate as describedpreviously.

FIG. 17 shows a third arrangement of the optical amplification unit.

In this figure, like reference numerals are used to denote correspondingparts to those in FIGS. 15 and 16 and their descriptions are omitted.

In FIG. 17, 143 denotes an optical splitter which branches part ofpumping light for control, 144; a photodiode which converts the pumpinglight branched by the optical splitter into an electrical signal, and145; a control circuit which receives the electrical signal produced bythe photodiode and performs necessary calculations to produce a controlsignal for the attenuator driver circuit 136.

This amplification unit is arranged to perform automatic power control(APC) to keep the amount of pumping light to the amplification mediumconstant. Part of pumping light branched by the optical splitter 143 isconverted into an electrical signal by the photodiode 144. Theelectrical signal is applied to the control circuit 145 where it iscompared with a preset value to produce a difference therebetween. TheAPC control is performed by controlling current to the variableattenuator 135 via the driver circuit 136 so that the difference willreach zero.

The pumping light having a constant level is coupled with incomingsignal light in the WDM coupler 132 to amplify the signal light in theamplification medium 131. The amplified signal light is applied to thewavelength selective filter 134 from which only the main signal light isderived for transmission over a light transmission path (not shown). Asdescribed previously, the optical isolator 133 is provided to avoidoscillation of light propagating in the reverse direction.

In this arrangement in which the power of pumping light is constant,when higher-power pumping light becomes necessary for system upgrade,the preset value in the control circuit 145 must be changed to asuitable value. However, if there is no need of increasing the power ofpumping light, the arrangement will function identically to the ALCcontrol and the AGC control.

FIG. 18 shows a fourth arrangement of the optical amplification unit.This arrangement is adapted for bidirectional pumping lightamplification. By varying current to each of variable attenuators 135through a corresponding one of driver circuits 136, the supply amount ofpumping light to the amplification medium from each of its input andoutput ends can be controlled individually. In the arrangement of FIG.18, two pumping light sources are used: one for pumping light supply tothe amplification medium from its input end, and one for pumping lightsupply from the output end.

It is desirable that a common pumping light source be used to supplypumping light to the amplification medium from its both ends. Anarrangement that uses such a common pumping light source is shown inFIG. 19.

The operation of each component for bidirectional excitation has beendescribed in conjunction with FIG. 5 and the description is thus omittedhere.

FIG. 19 shows a fifth arrangement of the optical amplification unit.

In FIG. 19, like reference numerals are used to denote correspondingparts to those in FIGS. 15 to 18 and their descriptions are omitted. Inthis figure, 146 denotes an optical splitter which splits pumping light.

The optical amplification unit of FIG. 19 has the same bidirectionalexcitation arrangement as the unit of FIG. 18 but differs in thatpumping light from a pumping light source is split by the opticalsplitter 146 and then input to the variable attenuators 135. Instead ofplacing the variable attenuators 135 as shown in FIG. 19, a singlevariable attenuator may be placed ahead of the optical splitter 146 tovary the amount of pumping light before it is split by the splitter. Inthis case, the ratio between the supply amount of pumping light to theamplification medium from its input end and the supply amount of pumpinglight from the output end depends on the dividing ratio in the opticalsplitter.

FIG. 20 shows an arrangement for operating pumping light sources stablyin the pumping light source unit of the optical amplifier.

In general, the connectors adapted for optical components, though havingvarying degrees of reflectiveness, reflect no little incoming light toproduce return light. When the return light falls on a pumping lightsource, its operation becomes unstable, causing the wavelength ofoscillating light to drift. This is because, when a pumping light sourceconsists of a laser which uses a resonator structure, the return lightupon incidence disturbs the resonant state of the resonator.

To solve such a problem, there are provided two pumping light sources157 and 158 of different wavelengths. The pumping light sources 157 and158 are respectively set to output light beams of λ1 and λ2 inwavelength which are linearly polarized perpendicular to each other. Theoutput beams of the light sources 157 and 158 are polarization-coupledby a polarization beam splitter (PBS) 159. A Faraday rotator 160 is setto rotate the plane of polarization of incoming light through 22.5degrees. After having been polarization-coupled, the pumping light hasits plane of polarization rotated through 22.5 degrees by the Faradayrotator 160 and then output from the pumping light source unit. Returnlight, resulting from the output pumping light being reflected fromoptical components within the optical amplifier, has its plane ofpolarization rotated through 22.5 degrees again in the Faraday rotator.It thus follows that the return light has its plane of polarizationrotated through 45 degrees with respect to the original light (theoutput light of the PBS). Thus, the return light will have wavelengthcomponents of λ1 and λ2.

Thus, if the return light has a component different in wavelength fromthe output light of an pumping light source, its action of disturbingthe resonant state within the resonator of a lager as the pumping lightsource will be suppressed. The operational instability of the pumpinglight source due to return light can therefore be eliminated.

That is, output light of pumping light sources (LDs) having differentoutput wavelengths is used as pumping light so that part of the pumpinglight that is reflected by optical components and falls on the pumpinglight source unit will not be uniform in wavelength. This allows returnlight to an LD to contain a wavelength component other than the outputwavelength of the LD, thus eliminating the operational instability ofthe optical amplifier.

FIG. 21 shows a fourth arrangement of the optical amplifier.

In this arrangement, optical amplification unit 170 is provided with apumping light input monitor 177 which comprises an optical splitter 174for branching part of pumping light, a photodiode 175 for receiving thepart of pumping light branched by the optical splitter, and a monitorcircuit 176 for monitoring the output level of the received pumpinglight. The other components of the optical amplification unit remainunchanged from those of FIG. 4 and their descriptions are thus omitted.In this arrangement, the output level of pumping light is monitored bythe pumping light input monitor 177 to activate an alarm, such as abuzzer or lamp, when the output level of pumping light has droppedabnormally.

In addition, when the input amount of pumping light to the opticalamplification unit 170 has dropped, the monitor circuit sends anelectrical signal to the pumping light source unit 171 to shut off thepumping light source unit. Alternatively, when the pumping light sourceunit has an output adjusting optical attenuator (not provided in thearrangement of FIG. 21), the attenuator is controlled to increase itslight attenuation amount to lower the output amount of pumping light tozero or a safety level.

Such control is performed by sending a control signal from the monitorcircuit 176 to a control circuit 173 in the pumping light source unit171, thereby causing light source drivers 172 to shut off pumping lightsources 43 or causing a variable attenuator, if provided, to increasethe light attenuation amount.

Such control is intended to detect a state where a connector 178 thatconnects the pumping light source unit 171 and the optical amplificationunit 170 together has become disconnected. Not only has the pumpinglight output from the pumping light source unit 171 very high power, butit is also converged by an optical fiber. If, when the connector hascome off, a person at work should be exposed to pumping light, thiswould entail great dangers to his or her skin and eyes. For this reason,the pumping light input monitor 177 is provided in the opticalamplification unit to detect the output level of the pumping light,thereby monitoring the state of the connector 178. When the output levelof the pumping light has dropped below a predetermined level, theconnector is considered to have become disconnected, whereupon the alarmis sounded or the pumping light source is shut down to keep the personat work from dangers.

FIG. 22 shows another arrangement for monitoring the connection betweenthe optical amplification unit and the pumping light source unit.

In this arrangement, light beams from multiple pumping light sources 183are first coupled and then split by an optical coupler/splitter 184. Thepumping light source unit is also connected with the opticalamplification unit by means of connectors 180. In this case as well, thestate of each connector is monitored.

That is, there is provided a reflection monitor 182, in which reflectedlight from the connector 180 is branched by an optical splitter 185 andthen detected by a photodiode 190 which sends the detected output to amonitor circuit 186. When the connector is off, the power of thereflected light from the connector becomes higher than when theconnector is connected. When the power of the reflected light has becomehigher than a preset level, the monitor circuit 186 produces an alarmsignal. Otherwise, when a variable attenuator is provided in the pumpinglight source unit, the monitor circuit controls the attenuator to reducethe output amount of pumping light to zero or a safety level.Alternatively, the monitor circuit 186 sends a control signal to acontrol circuit 187 to cause driver circuits 188 to shut off the pumpinglight sources 183.

FIGS. 23A and 23B illustrate how pumping light sources are connected inthe pumping light source unit.

In FIG. 23A, pumping light sources (laser diodes; LDs) are connected inseries with a transistor 200. This arrangement requires less transistorsand less power dissipation than when each of the LDs is drivenindividually.

As an example, consider a case where, as shown in FIG. 23B, one LD isattached to a transistor 201. Assuming that the voltage across the LD isV and the current flowing through the transistor is I, the powerdissipation in the transistor becomes P=(Vcc−V) I. On the other hand,when multiple LDs are connected in series as shown in FIG. 23A, thepower dissipation in the transistor 200 becomes P′=(V′cc−4V) I whereVcc=(1+α)V, V′cc=(4+α′) V, and α and α′ are assumed to be nearly equalto each other.

A comparison in transistor power dissipation between the arrangements ofFIGS. 23A and 23B indicates that 4P−P′=(4α−α′) and hence 4P is largerthan P′. Thus, a combination of series-connected LDs and one transistorrequires less power dissipation than with multiple combinations of eachof an LD and a transistor. Further, less transistors are required.

However, the pumping LDs need not be connected in series. For example,if pumping LDs are connected in parallel within the light source drivercircuits, a failing LD will be replaced with a good one more easily thanwith the series connection, providing redundancy.

The embodiments have been described in terms of an optical fiberamplifier. An optical semiconductor amplifier can also be used whichuses a semiconductor as the amplification medium.

The output power of pumping light to the amplification medium isadjusted by a variable attenuator, making it easy to adjust the outputpower of pumping light.

The optical amplifier is separated into the pumping light source unitcontaining a pumping light source or sources and the opticalamplification unit susceptible to heat, which allows the amplificationmedium in the optical amplification unit to be free from the influenceof heat emitted by the pumping light source and operate stably.

System upgrades, such as are intended to increase the signalmultiplexing degree, can be accommodated by adjusting the attenuationamount of the variable attenuator. By arranging the pumping light sourceunit so that an additional pumping light source or sources can beattached thereto, it becomes unnecessary to install redundant pumpinglight sources at the time of system installation, allowing the initialinvestment to be cut.

When pumping light is supplied to multiple amplification media from thepumping light source unit, the power of pumping light can be adjustedfor each amplification medium.

What is claimed is:
 1. A package incorporating multiple opticalamplification units each of which comprises an amplification medium foramplifying incoming signal light in response to application thereto ofpumping light supplied from an optical splitter means for splitting apumping light beam generated by one pumping light source into multiplelight beams or an optical coupler/splitter means for coupling pumpinglight beams generated by multiple pumping light sources into one beamand splitting the one beam into a plurality of beams, wherein a power ofthe pumping light is adjusted by a variable attenuator.
 2. An opticalamplifier for amplifying incoming signal light in response toapplication thereto of pumping light from a pumping light source unithaving a pumping light source for generating a pumping light beam andoptical coupler means for coupling multiple pumping light beams, thepumping light source unit comprising: a polarization plane rotating unitto rotate the plane of polarization of output pumping light from theoptical coupler means through a first angle of rotation for transmissionand rotating the plane of polarization of return light, resulting fromthe output pumping light being reflected from a connector connecting thepumping light source unit and other components of the optical amplifierback to the pumping light source unit, through a second angle ofrotation, thereby inputting to the pumping light source return lightdifferent in wavelength from the pumping light source generated by thepumping light source.
 3. An optical amplifier for amplifying incomingsignal light in response to application thereto of pumping light from apumping light source unit having multiple pumping light sources eachgenerating a pumping light beam and optical coupler/splitter means forcoupling multiple pumping light beams and splitting into a plurality ofbeams, the pumping light source unit comprising: a polarization planerotating unit to rotate the plane of polarization of output pumpinglight from the optical coupler/splitter means through a first angle ofrotation for transmission and to rotate the plane of polarization ofreturn light, resulting from the output pumping light being reflectedfrom a connector connecting the pumping light source unit and othercomponents of the optical amplifier together back to the pumping lightsource unit, through a second angle of rotation, thereby inputting tothe pumping light source return light different in wavelength from thepumping light source generated by the pumping light source.
 4. Thepackage according to claim 1, wherein the variable attenuator isprovided with the amplification medium in one package.
 5. The packageaccording to claim 1, wherein the variable attenuator is provided withthe optical splitter means or the optical coupler/splitter means in onepackage.
 6. An optical amplifier for amplifying incoming signal light inresponse to application thereto of pumping light from a pumping lightsource unit having a pumping light source for generating a pumping lightbeam and an optical coupler to couple multiple pumping light beams, thepumping light source unit comprising: a polarization plane rotating unitto rotate the plane of polarization of output pumping light from theoptical coupler through a first angle of rotation for transmission androtating the plane of polarization of return light, resulting from theoutput pumping light being reflected from a connector connecting thepumping light source unit and other components of the optical amplifierback to the pumping light source unit, through a second angle ofrotation, thereby inputting to the pumping light source return lightdifferent in wavelength from the pumping light source generated by thepumping light source.
 7. An optical amplifier for amplifying incomingsignal light in response to application thereto of pumping light from apumping light source unit having multiple pumping light sources eachgenerating a pumping light beam and an optical coupler/splitter tocouple multiple pumping light beams into a single beam and to split thesingle beam into a plurality of beams, the pumping light source unitcomprising: a polarization plane rotating unit to rotate the plane ofpolarization of output pumping light from the optical coupler/splitterthrough a first angle of rotation for transmission and to rotate theplane of polarization of return light, resulting from the output pumpinglight being reflected from a connector connecting the pumping lightsource unit and other components of the optical amplifier together backto the pumping light source unit, through a second angle of rotation,thereby inputting to the pumping light source return light different inwavelength from the pumping light source generated by the pumping lightsource.
 8. A device comprising: a connection device selected from thegroup consisting of an optical splitter to split a pumping light beamgenerated by a pumping light source into multiple light beams and anoptical coupler/splitter to couple pumping light beams generated bymultiple pumping light sources into one beam and to split the one beaminto a plurality of beams; multiple optical amplification units each ofwhich comprises an amplification medium for amplifying incoming signallight in response to application thereto of pumping light supplied fromconnection device; a package incorporating the multiple opticalamplification units; and at least one variable attenuator to adjust thepower of the pumping light applied to the optical amplification units.9. The device according to claim 8, wherein the at least one variableattenuator is provided with the multiple optical amplification units inthe package.
 10. The device according to claim 8, wherein the at leastone variable attenuator is provided with the connection device in apumping light unit.